feat(art): replace all 72 posts' artwork with the v2 pipeline

Every post now carries one unique local-ComfyUI/Ideogram 4 base frame, composited
into EN + ZH share cards, a banner and list thumbs.

- the shipped prompts_catalog held only 21 distinct metaphors for 72 posts (16
  shared one visual), so all 72 were rewritten as unique object+state metaphors
  and 10 frames re-rolled after OCR found pseudo-text rendered at legible size
- render_from_art.mjs emits JPEG by default (558 KB PNG -> 84 KB mozjpeg q85);
  the ZH 1200x630 card did not exist before this batch
- 288 frontmatter fields migrated; the 144 v1 PNGs are gone
- PostList's thumb helper keys off the base slug (a ZH card is zh-<slug>.jpg while
  the thumb is <slug>-800.webp, which 404'd the whole ZH listing)
- [...slug].astro's <picture> regex accepts .jpg
- the repo now carries the art inputs (metaphors.json, captions/<slug>.json,
  refreshed prompts_catalog.json) so a frame is reproducible from git alone

scripts/og-gen/captions/loop-engineering-without-a-coding-agent.json

@@ -0,0 +1 @@
+{"high_level_description":"A dark-mode 3D cyber-bento concept art and technical blueprint schematic of a single piece of industrial hardware. The main high-detail 3D focal subject is positioned strictly on the right side of the canvas, leaving expansive clean dark negative space across the entire left side.","style_description":{"aesthetics":"modern dark cyber-bento, industrial precision, sleek machined surfaces, raytraced ambient occlusion","lighting":"volumetric studio rim lighting in royal cobalt blue (#4d82ff) with emerald green (#4ade80) status nodes and gold (#f5c24a) telemetry glints and subtle cyan glow","medium":"3d_render","art_style":"octane render, matte obsidian hardware, frosted acrylic glass, technical blueprint vector lines","color_palette":["#0B0F19","#111827","#1E293B","#4ADE80","#F5C24A","#4D82FF"]},"compositional_deconstruction":{"background":"Deep midnight obsidian slate background (#0b0f19 to #111827) fading into pure clean negative space across the entire left 60% of the horizontal canvas, with faint technical blueprint grid crosshairs and delicate vector coordinate lines.","elements":[{"type":"obj","bbox":[150,550,850,945],"desc":"Eleven concentric circular obsidian gear loops rotating in mechanical harmony, eight solid pure crystal clockwork rings alongside three subtle neural pulse rings, operating autonomously. Clean unbranded matte surfaces, blank panels, no printed markings. Razor-sharp detail, no text, no letters, no watermark."}]}}